Hydraulic fracturing is commonly used to stimulate the production rate from subterranean wells. Fractures formed from fluid injection into the wells extend in a direction determined by stresses in the earth around the well. The fractures propagate in a direction normal to the minimum stress. At sufficient depth in the earth, the stress in the vertical direction is great enough to cause the fractures formed around wells by hydraulic pressure to be formed in a vertical direction in the earth.
The limit to vertical growth of such fractures is normally determined by an increase in horizontal stress or a change in mechanical properties in some strata in the earth. There is no known method to insure that a vertical fracture will not extend over a greater vertical interval than the subterranean zone which is to be stimulated in production rate by hydraulic fracturing, although some design variables can be selected to minimize the likelihood of "fracturing out of zone" in a hydraulic fracturing treatment. Models to predict the growth of vertical fractures are discussed at length in Recent Advances in Hydraulic Fracturing, SPE Monograph Vol. 12, Soc. of Pet. Engrs., Richardson, Tex., 1989, Chaps. 3, 4 and 5.
It is not unusual for multiple zones or beds penetrated by one well to be hydraulically fractured. The separate zones may be fractured simultaneously by having access from the wellbore, or they may be fractured sequentially by "stages," each stage isolating one segment of the wellbore and injecting fluids in the normal method. The separate stages are normally applied sequentially from the deeper to the shallower depths in a well. There is a question in such wells as to the vertical extent of the fracture formed in each stage. If the fracture from a stage applied deeper in the well influences a fracture formed in a shallower stage, the length of the fracture formed in the shallower stage is likely to be much shorter than expected. This may be caused by the much larger area for leak-off of fluid from the fracture and the possibility that zones having lower earth stress are contacted by the existing fracture.
Techniques have been developed in recent years to recover coal bed hydrocarbon gas from coal deposits. The gas, primarily methane, is produced by drilling wells and decreasing pressure in the coal to cause the methane to flow from the coal. Hydraulic fracturing has proven very helpful in increasing the production rate of the coal bed gas. Special techniques have been disclosed for forming and propping the fractures. U.S. Pat. No. 4,993,491 pertains to a method of injecting a range of sizes of proppant particles in a fracture in a coal bed. U.S. Pat. No. 4,665,990 discloses a method of alternating injection of fracturing fluid containing fine proppants and acidizing solution to fracture a subsurface coal formation.
There is a need for a method to increase the effectiveness of fractures when the initial fracture in a zone is improperly placed. Improper placement could be caused by stress not accounted for in the initial design or the influence of stimulations in other zones in the wellbore.